network as a critical mediator of epithelial-stromal interactions during normal mammary ductal development. Target genes for the hedgehog network include the FGF, TGFbeta, and Wnt superfamilies, among others, all of which have known roles in neoplastic progression as well as normal mammary gland development. Indeed, in other tissues, most notably skin, several genes in the hedgehog network have been identified as either oncogenes or tumor suppressor genes, but so far little is known about the involvement of this network in breast cancer. However, we have found that mice heterozygous for a targeted disruption of the Ptc1 gene, which encodes a receptor for the hedgehog family of ligands, develop hyperplasias similar to hyperplasias and ductal carcinomas in situ in the human breast. Regulation of these hyperplasias is hormone dependent, since they revert during pregnancy and lactation but return after involution. Together, these observations suggest the hypothesis that Ptc1 functions as a hormonally-regulated tumor suppressor gene in the breast. To test this hypothesis we propose three specific aims. In Aim 1 we will determine which function of Ptc1 is responsible for the defects observed by determining the mammary phenotype of Ptc1mes/Ptc1mes and Ptc1HhB/Ptc1HhB virgin mice. Ptc1mes is a newly identified hypomorphic allele of the Ptc1 gene. The protein encoded by this allele lacks most of the C-terminal cytoplasmic tail, and can therefore bind and sequester hedgehog ligand but lacks the ability to inhibit its downstream target SMO. The Ptc1HhB mutation eliminates hedgehog binding but allows PTC1 protein to inhibit SMO constitutively. Also in this aim, we will determine the null phenotype for Ptc1 in the epithelium by creating a mammary-specific conditional knock-out. In Aim 2 we will determine how hedgehog signaling status is altered by Ptc1 mutations and determine how these mutations affect hormone responses and other tissue-level processes. We will also investigate how hedgehog signaling changes as a function of neoplastic progression in the human breast. Finally, in Aim 3, we will determine whether Ptc1 acts as a mammary tumor suppressor gene by examining tumor formation in crosses with the MMTV-ErbB2 and C3-1-TAg transgenic mouse models of breast cancer.